Page 30 - A Practical Introduction to Optical Mineralogy

THE MICROSCOPIC STUDY OF MINERALS SYSTEMATIC DESCRIPTION OF MINERALS

I mm 1.6 Systematic description of minerals in polished section
using reflected light

Most of the ore minerals described in Chapter 3 have a heading
'polished section'. The properties presented under this heading are in a
particular sequence, and the terms used are explained bri.efly b~low. ~ ot
all properties are shown by each mineral, so only properties whtch mtght
be observed are given in Chapter 3.

1.6.1 Properties observed using plane polarised light (PPL)
The analyser is taken out of the optical path to give a bright image (see
Frontispiece).

Colour
Most minerals are only slightly coloured when observed using PPL, and
the colour sensation depends on factors such as the type of microscope,
Figure 1.7 Diagrammatic representation of a polished section of a sample of the light source and the sensitivity of an individ.ual's eyes. Colou~ is
lead ore. Transparent phases, e.g. fluorite (A), barite (B) and the mounting resin therefore usually described simply as being a vanety of grey or whtte,
(D) appear dark grey. Their brightness depends on their refractive index. The
e.g. bluish grey rutile, pinkish white cobaltite.
fluorite is almost black. Absorbing phases (opaque) , e.g. galena (C), appear
white. Holes, pits and cracks appear black. Note the black triangular cleavage pits
in the galena and the abundant pits in the barite which results, not from poor
polishing, but from the abundant fluid inclusions. Scratches appear as long Pleochroism
straight or curving lines. They are quite abundant in the galena which is soft and If the colour of a mineral varies from grain to grain and individual grains
scratches easily. change in colour on rotation of the stage, then the mineral is P.leochroic.
The colours for different crystallographic orientations are gtven when
(c) Holes, pits, cracks and specks of dust appear black. Reflection available. Covellite, for example, shows two extreme colours, blue and
from crystal faces in holes may give peculiar effects such as very bluish light grey. Pleochroism can often be observed only by careful
bright patches of light. examination of groups of grains in different crystallographic orientation.
(d) Scratches on the polished surface of minerals appear as long Alternatively the pleochroic mineral may be examined adjacent to a
straight or curving lines, often terminating at grain boundaries or non-pleochroic mineral, e.g. ilmenite against magnetite.
pits. Severe fine scratching can cause a change in the appearance of
minerals. Scratches on native metals, for example, tend to scatter
light and cause colour effects. l?efl ctance
(e) Patches of moisture or oil tend to cause circular dark or iridescent This is the percentage of light reflected from the polished surface of the
patches and indicate a need for cleaning of the polished surface. min rat, and where possible values are given for each crystallographi.c
(f) Tarnishing of minerals is indicated by an increase in colour inten- orientation. The eye is not good at estimating absolute reflectance butts
sity, which tends to be rather variable. Sulphides, for example u good comparator. The reflectance values of the minerals should there-
bornite, tend to tarnish rapidly. Removal of tarnishing usually fore be used for the purpose of comparing minerals. Reflectance can be
requires a few minutes buffing or repolishing. reluted to a grey scale of brightness in the following way, but although
(g) Polishing relief, due to the differing hardnesses of adjacent miner- followed in this book it is not a rigid scale. A mineral of reflectance
als, causes dark or light lines along grain contacts. Small soft bright - 15 % (e.g. phalerite) may appear to be light grey or white compared
grains may appear to glow, and holes may have indistinct dark with a low reflectance mineral (such as quartz) or dark grey compared
margins because of polishing relief. with a bright mineral (such as pyrite):
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